The invention relates to a method for manufacturing biodegradable products, to an apparatus which can be used therefor and to products obtainable according to this method. The products to be manufactured according to the invention have a foamy structure. More particularly, the foamy material always comprises at least three parts: two relatively dense layers on the outside, which as it were, form a skin, and between them a foam structure as core. The dense layers are firm and strong and consist of substantially closed, small cells. The foam structure of the core is generally open, which means that the cells have burst to allow the gases evolving during the manufacture, for instance water vapor or carbon dioxide gas, to escape. The cells generally have a firm and solid cell wall due to the relatively high pressure and temperature during the process.
In this description, "gelatinization" is understood to mean a change of a natural polymer from a slightly or completely loose granular or comparable granulate form into a cohesive form which may or may not be dry and/or foamed, in which stretched polymers are present which are mutually bonded to a limited extent, if at all. That is to say, a transition occurs from a solid substance, a colloidal solution or suspension to a more homogeneous fluid mass. Depending on the polymers used, "gelatinization" should therefore be understood to include, for instance, gelling, gellating and the like.
In foamed products where only gelatinization occurs, as a result of gas evolution, bubbles are formed in the mass to be foamed, substantially after gelatinization. This process occurs at relatively low temperatures and pressures. Over the entire cross section, such products have approximately the same structure of relatively small cells with walls of substantially uncross-linked natural polymers.
In this description, "baking" is understood to mean a method in which both gelatinization and cross-linking occur, at relatively high temperature and/or pressure. As a result, the formation of gas arises relatively soon, so that bubbles are already formed prior to or during gelatinization. As a result of inter alia the high pressure adjacent strongly heated parts, the polymers cross-link quickly when using a mold or like baking form with a temperature at or above the baking temperature.
These baked products have a core with relatively large cells, enclosed between skin parts with relatively small cells. The cell walls have a relatively high density and the natural polymers included therein are cross-linked to a high extent, which means that they have entered into mutual chain bonds. Such a baked product therefore has a sandwich-like structure.
International patent application 91/12186 discloses a method for manufacturing biodegradable products by heating in a baking mold a batter which comprises at least natural polymers in the form of starch or derivatives thereof. The batter is introduced into an open platen set, for instance, a wafer iron, whereafter the platen set is closed and the batter is "baked". This results in a thin-walled product which is biodegradable and yet firm and is relatively well resistant to moisture, at least as long as the skin of the product is not damaged. The product is ready immediately and so requires no post-treatment. Owing to the heating to relatively high temperatures, a structure of blown cells and cross-linked starch is created in the product. The products are relatively cheap to manufacture, have good storing properties under different conditions, are light and convenient in use and, owing to their biodegradability, are environment-friendly.
A disadvantage using of platen sets is that the batter is introduced into an open mold which is subsequently closed and, for instance, is passed through a continuous oven, where it is heated, for instance by gas burners. Energetically speaking, this is not very efficient and moreover the temperature in the baking mold is not properly controllable and may vary considerably during the baking process, which is disadvantageous to the quality of the products. Moreover, the products which are obtained according to this method are not particularly dimensionally stable and allow no or only very slight differences in wall thickness, because otherwise no homogeneous structure can be obtained. A further disadvantage of this method is that the introduction of the batter and the removal of the product is very laborious and will often lead to failure in the production. Moreover, with this method no products can be manufactured that are non-withdrawable, so that the molding freedom is limited.
European patent application 0 512 589 discloses a method for making thin-walled biodegradable products, in which platen sets are likewise used. In this known method, a starch-containing dough is introduced into an open mold cavity in one of the platens, whereafter the platen set is closed and is subsequently heated to a temperature at which only gelatinization occurs but at which the dough is not "baked". The temperature is therefore kept relatively low with respect to the previously described method. With this method, products are obtained which are directly ready for use, that is, they do not require any post-treatment. It is true of the products obtained by this procedure too that they have little dimensional stability and permit no, or only very slight, differences in wall thickness in order to preserve a homogeneous structure. Since the products are not "baked", they are less stiff and exhibit relatively poor resistance to, for instance, water and varying temperature conditions. Moreover, it is true of this method too that filling the platen sets and removing the products is cumbersome and time-consuming, that the products can easily be damaged when being removed and cannot be non-withdrawable, so that the freedom in the molding design is limited.
International patent application 93/08014 discloses a method for manufacturing biodegradable products, in which the products are manufactured by extrusion of a mixture comprising at least starch or derivatives thereof. In this method, a dry, crude starch with less than 30% water is mixed with mild acid, which mixture is stirred with a carbonate which, through reaction with the acid, can give rise to CO.sub.2 gas. This mixture is introduced into an extrusion tank and mixed with water, while being pressurized and heated to such an extent as to give rise to gelatinization of the starch. In the extrusion tank the acid is reacted with the gelatinized starch, in such a manner that the average molecular weight thereof decreases and the uniform bonds of the starch chains are broken, while moreover, through reaction with the carbonate, CO.sub.2 gas is produced for blowing up the modified starch. The thus obtained mixture of blown starch with altered (micro) structure is thereafter forced through an extrusion die, whereby under the influence of the CO.sub.2 gas a closed-cell structure is obtained with a density of less than 0.032 g per cubic centimeter. Owing to this structure, the thus obtained product has elastic properties and permits of rapid biological decomposition.
A disadvantage of this known method is that the raw materials are to be supplied in relatively dry form and in the extrusion tank, are to be mixed with water under simultaneous increase of the temperature in the tank, whereby the desired gelatinization occurs. To that end, the mixture must be heated, which is difficult to effect homogeneously in view of the relatively large mass. As a consequence, the process is relatively poorly controllable. A further disadvantage is that the products obtained in this way have only limited durability and are not water-resistant and moreover are not particularly dimensionally stable. As a result of the extrusion process and the strong expansion occurring after the extrusion, the freedom of design in this method is limited.
European patent application 0 118 240 discloses a method for manufacturing biodegradable medicament capsules and like products by injection-molding from a starch composite. To that end, a starch mixture with a low water content is introduced into a closed space, in particular the hopper of an injection-molding machine, where plasticization of the mixture is provided for at a suitable specific temperature, pressure and humidity. The temperature and pressure are increased to such an extent that the mixture is adjusted to above the vitrification point. Thereafter the plasticized mixture is forced into a cold mold and maintained under pressure, until the, or each, product has cooled off sufficiently, whereafter the mold is opened and emptied.
The advantage of this known method is that dimensionally stable biodegradable products can be manufactured relatively fast. However, the possible dimensions of products that can be manufactured with this method are limited, owing to the flow path in the mold. In fact, the plasticized mass forced into the mold is cooled directly, which gives rise to solidification and prevents flow into the mass relatively soon after entry of the mold. Moreover, no cross-linking of the starch in the mass occurs, so that the products have relatively weak strength properties and exhibit relatively poor resistance to water and moist conditions in general. In a moist environment the products will take up a great deal of water and thereby become slack; conversely, in a dry environment moisture will evaporate from the products, so that they become hard and brittle. The products obtained with this method have a high density and have no foamy structure.
European patent application 0 407 350 proposes an improved composition of starch composites for use in inter alia a method according to the above-described European patent application 0 118 240, by which, for instance through casting or extrusion, products can be manufactured with better strength properties and better resistance to different conditions, in particular as regards humidity and temperatures. To that end, to the composite of starch, a thermoplastic plastic is added, whereafter the mixture is transformed under very well regulated and controlled conditions into a melt allowing subsequent casting or extrusion. By controlling the conditions, it can be ensured that the thermoplastic plastic melts and mixes with the starch without the starch disintegrating.
An advantage of this known method is that the products are dimensionally stable, have good strength properties and exhibit relatively good resistance to humidity and temperature fluctuations. One of the disadvantages of this known method is that the thermoplastic plastic must be added. This reduces the biodegradability and it is less attractive from an environmental point of view. Further, these products too have a relatively high density. Moreover, when using the composite for injection molding, the above-mentioned drawbacks remain, such as, for instance, the laborious and costly preparation of the composite, the limited possibilities as regards dimensioning and the absence of cross-linking of the natural polymers.
International patent application 95/04104 discloses a method for manufacturing foamed, biodegradable products from starch-containing raw materials, in which an amount of starch is liquified in a pre-stage by heating to a temperature far above the gelatinization temperature, whereafter an amount of water-saturated ramie fibers is admixed. This mixture is thereafter passed into or through a mold or converted to a dry granulate. Upon heating of the mixture, the water is to escape from the ramie fibers and to function as blowing agent. When using this known method, a substantially dry granulate of starch is to be strongly heated in the pre-stage, which granulate therefore cannot form a liquid batter. Accordingly, this method suffers from the above-mentioned disadvantages of the gelatinization of the mass prior to its introduction into the mold, such as for instance the laborious and costly preparation of the composite, the limited possibilities as regards to dimensioning.
International patent application 92/13004 discloses a method for manufacturing solid and foamed, biodegradable products from starch-containing raw materials. In this apparatus, an amount of moist (20% water) starch is mixed with inter alia some water and mixed in a heated vat in order to obtain gelatinization of the mass, whereafter it is processed through an outlet opening into film or sheet. Thereupon, the film can be deformed into, for instance, dish products. In this known method too, the liquidity of the suspension to be processed is obtained by heating the mass, to above the vitrification temperature prior to the definitive processing. Moreover, to the heated mass an amount of steam or alcohol vapor is added. The processing means (for instance rollers) are maintained at a relatively low temperature (70.degree. C.). Further, European patent application 0 634 261 discloses a method for manufacturing biodegradable products utilizing a kind of injection-molding technique, which starts from a mixture of a first and second biodegradable starting material. The first has a melting temperature of above 100.degree. C., the second of less than 100.degree. C. Either a substance which contains water is added to the starting material, or water is incorporated in the starting material, in such a manner that it can provide for the blowing of the cells. In an extruder press, the mass is heated to above the gelatinization temperature of at least the first starting material, mixed and pressurized and subsequently sprayed into a mold cavity provided in a pressurized space. After introduction of the mass, the pressure is removed, so that the water in the mass expands, blows the cells and exits through the permeable wall of the mold cavity. Such a method requires a complicated composition of starting materials, which moreover are not entirely biodegradable. Further, this known method has the above-mentioned disadvantages resulting from the gelatinization of at least a part of the mass prior to its introduction into the mold, such as for instance the laborious and costly preparation of the composite, the limited possibilities as regards to dimensioning. In particular, as a result of inter alia the porous walls, the outer wall portions of the products manufactured according to this method will not have a dense, compact wall but a uniform distribution of cells of uniform size throughout the product thickness.